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25.6 Cyclic Forms of Carbohydrates: Furanose Forms
43
25.6 25.6 Cyclic Forms of Carbohydrates: Cyclic Forms of Carbohydrates: Furanose Forms Furanose Forms
Transcript of 25.6 Cyclic Forms of Carbohydrates: Furanose Forms
25.625.6
Cyclic Forms of Carbohydrates:Cyclic Forms of Carbohydrates:
Furanose FormsFuranose Forms
Recall from Section 17.8Recall from Section 17.8
R"OR"OHHCC••••OO ••••
RR
R'R'
R"OR"O CC OO HH••••
••••
••••
••••
RR
R'R'
Product is a hemiacetal.Product is a hemiacetal.
++
Cyclic HemiacetalsCyclic Hemiacetals
Aldehydes and ketones that contain an Aldehydes and ketones that contain an OOH H group elsewhere in the molecule can undergo group elsewhere in the molecule can undergo intramolecular hemiacetal formation.intramolecular hemiacetal formation.The equilibrium favors the cyclic hemiacetal if The equilibrium favors the cyclic hemiacetal if the ring is 5the ring is 5-- or 6or 6--membered.membered.
CC OO
RR
OOHH
CC
OHOHRR
OO
equilibrium lies far to the rightequilibrium lies far to the rightcyclic hemiacetals that have 5cyclic hemiacetals that have 5--membered ringsmembered ringsare called are called furanosefuranose formsforms
Carbohydrates Form Cyclic HemiacetalsCarbohydrates Form Cyclic Hemiacetals
CHCH OO
CHCH22OOHH
11
22
33
44
HH
OHOHOO
11
2233
44
stereochemistry is maintained during cyclicstereochemistry is maintained during cyclichemiacetal formationhemiacetal formation
DD--ErythroseErythrose
CHCH OO
CHCH22OOHH
11
22
33
44
HH
OHOHOO
11
2233
44
HH
HH
OHOH
OHOH
HH HH HH
OHOHOHOHHH
D-Erythrose
1
2
3
4
turn 90°
D-Erythrose
1
23
4
1
2
3
4
D-Erythrose
move O into position by rotating about bond between carbon-3 and carbon-4
1
23
4
D-Erythrose
1
23
41
23
4
D-Erythrose
1
23
4close ring by hemiacetal formation between OH at C-4 and carbonyl group
D-Erythrose
1
23
41
23
4
stereochemistry is variable at anomeric carbon;stereochemistry is variable at anomeric carbon;two diastereomers are formedtwo diastereomers are formed
DD--ErythroseErythrose
CHCH OO
CHCH22OOHH
11
22
33
44
HH
OHOHOO
11
2233
44
HH
HH
OHOH
OHOH
HH HH HH
OHOHOHOHHH
anomeric carbonanomeric carbon
DD--ErythroseErythrose
HH
OHOHOO
11
2233
44
HH HH HH
OHOHOHOHHH
OHOH
HHOO
11
2233
44
HH HH HH
OHOHOHOHHH
αα--DD--ErythrofuranoseErythrofuranose ββ--DD--ErythrofuranoseErythrofuranose
DD--RiboseRibose
CHCH OO
CHCH22OHOH
HH OOHHHH OHOHHH OHOH
11
22
33
44
55
furanose ring formation involves furanose ring formation involves OOH group at CH group at C--44
DD--RiboseRibose
CHCH OO
CHCH22OHOH
HH OOHHHH OHOHHH OHOH
11
22
33
44
55
need C(3)need C(3)--C(4) bond rotation to put C(4) bond rotation to put OOH in proper H in proper orientation to close 5orientation to close 5--membered ringmembered ring
CHCH OOHHHH
HH CHCH22OHOH
OHOHOHOHHHOO
11
2233
44
55
DD--RiboseRibose
CHCH OOHHHH
HH CHCH22OHOH
OHOHOHOHHHOO
11
2233
44
55
CHCH OOHHHH
HH
HOCHHOCH22
OHOHOHOH
OOHH11
2233
44
55
DD--RiboseRibose
CHCH22OH group becomes a substituent on ringOH group becomes a substituent on ring
CHCH OOHHHH
HH
HOCHHOCH22
OHOHOHOH
OOHH11
2233
44
55
DD--RiboseRibose
CHCH22OH group becomes a substituent on ringOH group becomes a substituent on ring
CHCH OOHHHH
HH
HOCHHOCH22
OHOHOHOH
OOHH11
2233
44
55 HOCHHOCH22
HH
OHOHOO
11
2233
44HH HH
OHOHOHOHHH
55
ββ--DD--RibofuranoseRibofuranose
25.725.7
Cyclic Forms of Carbohydrates:Cyclic Forms of Carbohydrates:
Pyranose FormsPyranose Forms
cyclic hemiacetals that have 6cyclic hemiacetals that have 6--membered ringsmembered ringsare called are called pyranosepyranose formsforms
Carbohydrates Form Cyclic HemiacetalsCarbohydrates Form Cyclic Hemiacetals
HH
OHOHOO11
2233
44
55
CHCH OO
CHCH22OOHH
11
22
33
44
55
DD--RiboseRibose
CHCH OO
CHCH22OOHH
11
22
33
44
55
HH
HH
HH OHOH
OHOH
OHOH
pyranose ring formation involves pyranose ring formation involves OOH group at CH group at C--55
DD--RiboseRibose
CHCH OOHHHH
HH CHCH22OOHH
OHOHOHOHHOHO
11
2233
44
55CHCH OO
CHCH22OOHH
11
22
33
44
55
HH
HH
HH OHOH
OHOH
OHOH
pyranose ring formation involves pyranose ring formation involves OOH group at CH group at C--55
DD--RiboseRibose
CHCH OOHHHH
HH CHCH22OOHH
OHOHOHOHHOHO
11
2233
44
55
pyranose ring formation involves pyranose ring formation involves OOH group at CH group at C--55
DD--RiboseRibose
CHCH OOHHHH
HH CHCH22OOHH
OHOHOHOHHOHO
11
2233
44
55
HH
OHOHOO11
2233
44
OHOHOHOHHOHO
HHHH
HHHH
HH55
ββ--DD--RibopyranoseRibopyranose
DD--RiboseRibose
HH
OHOHOO11
2233
44
OHOHOHOHHOHO
HHHH
HHHH
HH55
αα--DD--RibopyranoseRibopyranose
OHOH
HHOO11
2233
44
OHOHOHOHHOHO
HHHH
HHHH
HH55
DD--GlucoseGlucose
CHCH OO
CHCH22OHOH
11
22
33
44
55
HH
HOHO
HH OHOH
HH
OHOH
HH OOHH66
pyranose ring formation involves pyranose ring formation involves OOH group at CH group at C--55
DD--GlucoseGlucose
CHCH OO
CHCH22OHOH
11
22
33
44
55
HH
HOHO
HH OHOH
HH
OHOH
HH OOHH66
OOHH
CHCH OOHHOHOH
HH CHCH22OHOH
OHOHHHHOHO
11
2233
44
5566
HH
pyranose ring formation involves pyranose ring formation involves OOH group at CH group at C--55
DD--GlucoseGlucose
CHCH OOHH
HH CHCH22OHOH
OHOHHHHOHO
11
2233
44
5566
HH
OOHHOHOH
need C(4)need C(4)--C(5) bond rotation to put C(5) bond rotation to put OOH in proper H in proper orientation to close 6orientation to close 6--membered ringmembered ring
DD--GlucoseGlucose
CHCH OOHHOHOH
HH CHCH22OHOH
OHOHHHHOHO
11
2233
44
5566
HH
OOHH
CHCH OOHHOHOHHH
HOCHHOCH22
OHOHHHHOHO
11
2233
4455
66
HHOOHH
need C(4)need C(4)--C(5) bond rotation to put C(5) bond rotation to put OOH in proper H in proper orientation to close 6orientation to close 6--membered ringmembered ring
DD--GlucoseGlucose
CHCH OOHHOHOHHH
HOCHHOCH22
OHOHHHHOHO
11
2233
4455
66
HHOOHH
ββ--DD--GlucopyranoseGlucopyranose
HH
OHOHOO11
2233
44
OHOHHHHOHO
HHOHOH
HHHH
HOCHHOCH22
55
66
DD--GlucoseGlucose
ββ--DD--GlucopyranoseGlucopyranose
HH
OHOHOO11
2233
44
OHOHHHHOHO
HHOHOH
HHHH
HOCHHOCH22
55
66
αα--DD--GlucopyranoseGlucopyranose
OHOH
HHOO11
2233
44
OHOHHHHOHO
HHOHOH
HHHH
HOCHHOCH22
55
66
DD--GlucoseGlucose
ββ--DD--GlucopyranoseGlucopyranose
HH
OHOHOO11
2233
44
OHOHHHHOHO
HHOHOH
HHHH
HOCHHOCH22
55
66
pyranose forms of carbohydrates adopt chair pyranose forms of carbohydrates adopt chair conformationsconformations
DD--GlucoseGlucose
ββ--DD--GlucopyranoseGlucopyranose
HH
OHOHOO11
2233
44
OHOHHHHOHO
HHOHOH
HHHH
HOCHHOCH22
55
66
OHOH
HHOHOH
HH
HOHOHOHO
HH
HHHH
HOCHHOCH22
OO
all substituents are equatorial in all substituents are equatorial in ββ--DD--glucopyranoseglucopyranose
112233
4455
66
DD--GlucoseGlucose
αα--DD--GlucopyranoseGlucopyranose
OHOH
HHOHOH
HH
HOHOHOHO
HH
HHHH
HOCHHOCH22
OO
OH group at anomeric carbon is axialOH group at anomeric carbon is axialin in αα--DD--glucopyranoseglucopyranose
11
ββ--DD--GlucopyranoseGlucopyranose
HH
OHOHOHOH
HH
HOHOHOHO
HH
HHHH
HOCHHOCH22
OO
11
Figure 25.5Figure 25.5
CHCH OO
CHCH22OHOH
HH OOHHHH OHOHHH OHOH
Less than 1% of the openLess than 1% of the open--chain form of chain form of DD--ribose ribose is present at equilibrium in aqueous solution.is present at equilibrium in aqueous solution.
Figure 25.5Figure 25.5
OHOH
HHOHOH
HH
HHHOHO
HH
OHOHHH
OO
ββ--DD--Ribopyranose (56%)Ribopyranose (56%)
HH
HOHO
αα--DD--Ribopyranose (20%)Ribopyranose (20%)
HH
OHOHOHOH
HH
HH
HH
OHOHHH
OO
11
HH
76% of the 76% of the DD--ribose is a mixture of the ribose is a mixture of the αα and and ββ--pyranose forms, with the pyranose forms, with the ββ--form predominatingform predominating
Figure 25.5Figure 25.5
HOCHHOCH22
HH
OHOHOOHH HH
OHOHOHOHHH
ββ--DD--Ribofuranose (18%)Ribofuranose (18%)
HOCHHOCH22
OHOH
HHOOHH HH
OHOHOHOHHH
αα--DD--Ribofuranose (6%)Ribofuranose (6%)
The The αα and and ββ--furanose forms comprise 24% of furanose forms comprise 24% of the mixture.the mixture.
25.825.8MutarotationMutarotation
MutarotationMutarotation
Mutarotation is a term given to the change in Mutarotation is a term given to the change in the observed optical rotation of a substance with the observed optical rotation of a substance with time. time.
Glucose, for example, can be obtained in Glucose, for example, can be obtained in either its either its αα or or ββ--pyranose form. The two forms pyranose form. The two forms have different physical properties such as have different physical properties such as melting point and optical rotation.melting point and optical rotation.
When either form is dissolved in water, its When either form is dissolved in water, its initial rotation changes with time. Eventually initial rotation changes with time. Eventually both solutions have the same rotation.both solutions have the same rotation.
Mutarotation of Mutarotation of DD--GlucoseGlucose
αα--DD--GlucopyranoseGlucopyranose
OHOH
HHOHOH
HH
HOHOHOHO
HH
HHHH
HOCHHOCH22
OO
11
ββ--DD--GlucopyranoseGlucopyranose
HH
OHOHOHOH
HH
HOHOHOHO
HH
HHHH
HOCHHOCH22
OO
11
Initial: [Initial: [αα]]DD +18.7°+18.7° Initial: [Initial: [αα]]DD +112.2°+112.2°
Mutarotation of Mutarotation of DD--GlucoseGlucose
αα--DD--GlucopyranoseGlucopyranose
OHOH
HHOHOH
HH
HOHOHOHO
HH
HHHH
HOCHHOCH22
OO
11
ββ--DD--GlucopyranoseGlucopyranose
HH
OHOHOHOH
HH
HOHOHOHO
HH
HHHH
HOCHHOCH22
OO
11
Initial: [Initial: [αα]]DD +18.7°+18.7° Initial: [Initial: [αα]]DD +112.2°+112.2°
Final: [Final: [αα]]DD +52.5°+52.5°
Mutarotation of Mutarotation of DD--GlucoseGlucose
αα--DD--GlucopyranoseGlucopyranose
OHOH
HHOHOH
HH
HOHOHOHO
HH
HHHH
HOCHHOCH22
OO
11
ββ--DD--GlucopyranoseGlucopyranose
HH
OHOHOHOH
HH
HOHOHOHO
HH
HHHH
HOCHHOCH22
OO
11
Initial: [Initial: [αα]]DD +18.7°+18.7° Initial: [Initial: [αα]]DD +112.2°+112.2°
Final: [Final: [αα]]DD +52.5°+52.5°
Mutarotation of Mutarotation of DD--GlucoseGlucose
αα--DD--GlucopyranoseGlucopyranose
OHOH
HHOHOH
HH
HOHOHOHO
HH
HHHH
HOCHHOCH22
OO
11
ββ--DD--GlucopyranoseGlucopyranose
HH
OHOHOHOH
HH
HOHOHOHO
HH
HHHH
HOCHHOCH22
OO
11
Explanation: After being dissolved in water, the Explanation: After being dissolved in water, the αα and and ββ forms slowly interconvert via the openforms slowly interconvert via the open--chain form. An equilibrium state is reached that chain form. An equilibrium state is reached that contains 64% contains 64% ββ and 36% and 36% αα. .
